Fabrication, characterization, and high temperature surface enhanced Raman spectroscopic performance of SiO2 coated silver particles

Ming Liu a, Rong Xiang *a, Yaerim Lee a, Keigo Otsuka a, Ya-Lun Ho a, Taiki Inoue a, Shohei Chiashi a, Jean-Jacques Delaunay a and Shigeo Maruyama *ab
aDepartment of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan. E-mail: maruyama@photon.t.u-tokyo.ac.jp; xiangrong@photon.t.u-tokyo.ac.jp
bEnergy NanoEngineering Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8564, Japan

Received 19th November 2017 , Accepted 4th February 2018

First published on 5th February 2018

We present a systematic study on the fabrication, characterization and high temperature surface enhanced Raman spectroscopy (SERS) performance of SiO2 coated silver nanoparticles (Ag@SiO2) on a flat substrate, aiming to obtain a thermally robust SERS substrate for monitoring high temperature reactions. We confirm that a 10–15 nm SiO2 coating provides a structure stability up to 900 °C without significantly sacrificing the enhancement factor, while the uncoated particle cannot retain the SERS effect above 500 °C. The finite difference time domain (FDTD) simulation results supported that the SiO2 coating almost has no influence on the distribution of the electric field but only physically trapped the most enhanced spot inside the coating layer. On this thermally robust substrate, we confirmed that the SERS of horizontally aligned single walled carbon nanotubes is stable at elevated temperatures, and demonstrate an in situ Raman monitoring of the atmosphere of the annealing process of nanodiamonds, in which the interconverting process of C–C bonds is unambiguously observed. We claim that this is a first experimental proof that the high temperature SERS effect can be preserved and applied in a chemical reaction at temperature above 500 °C. This versatile substrate also enables novel opportunities for observing growth, etching, and structure transformation of many 0D and 2D nano-materials.


Surface enhanced Raman spectroscopy (SERS), discovered by Jeanmaire and Van Duyne on roughened Ag electrodes in 1977,1 has attracted exponentially increasing attention because of its ability to provide non-destructive and ultra-sensitive characterization of a wide variety of analytes at extremely low concentrations.2 It has been found to be an important spectroscopy approach in identifying chemical species and obtaining structural information in many fields like biosensing,3–5 chemical detection,6,7 and electrochemistry.8 Jeanmaire and Van Duyne, and Albrecht and Creighton have proved that an enhancement in the Raman signal was due to a localized electromagnetic field around the metallic nanostructures (e.g. Au, Ag and Cu).1,9,10 Typically, different metals are SERS effective at different wavelengths, and silver nanostructures are considered as the most common SERS active species due to their unique plasmonic properties, which lead to a higher enhanced effect of the Raman spectroscopy compared to other materials in the visible range.10

Due to the high sensitivity in detecting the structure change, one possible application field for SERS is in situ monitoring a catalytic reaction. However, many catalytic reactions need a high temperature environment while almost all previous research on SERS are carried out at room temperature only. There are still a number of limitations associated with the current SERS field at elevated temperatures. For example, nanostructures rapidly lose their enhancement capabilities in a short period of time under high-temperature conditions due to surface oxidation (for Ag) and the aggregation process.11–14 Moreover, high temperature SERS theory has not been intensively explored, although there are several theoretical studies predicting that the SERS enhancement ratio decreases at elevated temperature at scattering frequencies close to the surface plasmon resonance.15,16 However, if the excitation wavelength is above the surface plasmon resonance, the SERS effect is relatively insensitive to temperature.15 Therefore, a prerequisite for successful applications of SERS in monitoring catalyst reaction is to maintain the particles' structural morphology at high temperatures and investigate the high temperature SERS mechanism.

One studied system of focused research interest is catalytic synthesis, or transformation of nano-carbon materials. Though SERS is extremely sensitive in detecting molecules of biomedical or national security interest,17–20 it has received only limited attention in the study of catalysis,21–23 especially real time monitoring of a catalyst reaction like the growth process of nano-carbon materials, e.g. single-walled carbon nanotubes (SWNTs) and nano-diamonds. There are several reports on in situ monitoring Raman scattering of SWNTs during the growth process. For example, Chiashi et al. revealed that the G-band intensity increased nearly linearly with time after the initial rapid increase during the growth process;24 Navas et al. investigated the evolution of nanotube diameter distribution during the growth by in situ Raman scattering.25 In general, however, the G-band Raman signal obtained at a high temperature is always too weak and broad to explicitly recognize the structure-related fingerprints.26–28 In our previous work,29 we proposed to use silver nanoparticles to enhance the Raman scattering of SWNTs during growth, and confirmed that the SERS effect of silver nanoparticles was stable and insensitive to elevated temperature at the same morphology. The drawback of this previous strategy is the morphology degradation at high temperature leading to a significant decrease in the SERS signal, i.e. the thermal stability of metal particles became the limitation for further using SERS in monitoring a high temperature reaction.

In this work, we are devoted to improving the high temperature stability of the SERS substrate by sputtering a thermally robust silicon dioxide (SiO2) thin layer on Ag nanoparticles to form a structure consisting of an Ag core capped by a SiO2 shell. We perform a systematic investigation on the fabrication, structure optimization, and SERS characterization of Ag@SiO2 at room and high temperatures. A sub 15 nm SiO2 coating is proven to be able to provide a structure stability up to 900 °C without significantly sacrificing the enhancement factor. A clear and thermally stable SERS enhancement was observed during the heating process of horizontally aligned (HA-) SWNTs. Furthermore, in situ monitoring the thermal annealing of nanodiamonds with few orders of magnitude enhancement is demonstrated on these SERS substrates. Finally, finite difference time domain (FDTD) simulation provided a verification about the SERS mechanism of this structure.

Results and discussion

Fig. 1a shows a schematic of our strategy using SiO2 coating to prevent the migration and aggregation of silver nanoparticles at high temperatures. The choice of SiO2 is not only because of its high melting point that can enhance the thermal stability of the nanoparticles, but also its chemical inertness and capability of supporting the growth of many nano-materials including SWNTs. Representative scanning electron microscopy (SEM) images and the corresponding dark field images (insets) of the pure Ag, 10 nm SiO2 coated Ag (Ag@10 nm SiO2), and 50 nm SiO2 coated Ag (Ag@50 nm SiO2) are shown in Fig. 1c, d, and e, respectively. It is clearly seen in SEM images that the morphology of each type of SERS substrate is uniform and silver nanoparticles are isolated on the substrate. IV characterization (Fig. S1) confirms that the three SERS substrates are insulators, suggesting that the particles are well isolated on the substrate. The dark field images reveal strong scattering by the structure interpreted as SERS hot spots. The synthesis of SWNTs was performed on Ag@10 nm SiO2 to prove the capability of this sputtering SiO2 layer to support the growth of nano-materials. The representative SEM image and Raman spectra of SWNTs synthesized on the SERS substrate are shown in Fig. S2.
image file: c7nr08631h-f1.tif
Fig. 1 (a). Schematic illustration of Ag nanoparticle coated by SiO2 on the substrate under laser excitation; (b) schematic illustration of sectional view of Ag nanoparticle coated by SiO2 on the substrate; (c–e) representative SEM images of Ag particles, Ag coated by 10 nm SiO2, Ag coated by 50 nm SiO2, and the corresponding dark field images (inset); (f) histogram of diameter distribution of Ag nanoparticles inside the SiO2 coating layer and a representative TEM image.

To obtain the detailed morphology and dimension of SiO2 coated particles, we perform transmission electron microscopy (TEM) characterization using a Si/SiO2 TEM grid. The fabrication procedure of SiO2 coated nanoparticles on the grid is the same as that on a normal silicon wafer, which guarantees that particles on the TEM grid possess the same surface morphology and particle size. The diameters of around 200 particles are manually measured in TEM images, and the histogram is shown in Fig. 1f. Although we realize that Ag nanoparticles are not perfectly spherical in shape and there is a slight size difference among batches (approx. 50%, seen Fig. S3), the measurement method is kept consistent among all samples and representative data are used to demonstrate the tendency. A typical TEM image of the particle is shown in Fig. 1f, which suggests that the average diameter of the inside silver particle is 7.2 nm and 49% of the particles are distributed in the range from 2 nm to 6 nm. This histogram of diameter distribution was obtained by manually measuring the diameter of around 200 particles in TEM images in a consistent approach. The Ag nanoparticles inside SiO2 coating layer are not in perfect spherical shapes and there is a slightly size difference among batches (approx. 50%) which is demonstrated by TEM and SEM images in Fig. S3. After the characterization of SERS substrates, Rhodamine B was utilized to confirm the enhancement effect of these substrates.

A systematic comparison of the SERS effect before and after coating SiO2 is presented in Fig. 2, by detecting standard Rhodamine B ethanol solution with a concentration of 5.3 × 10−4 M. The four wavelengths 488 nm, 532 nm, 633 nm, and 785 nm were used to excite the Raman scattering of rhodamine B on four types of substrates including silicon wafer, pure Ag particles, Ag@10 nm SiO2 and Ag@50 nm SiO2. There is almost no obvious Raman peak of Rhodamine B on the silicon wafer (black lines in four graphs), compared with the significantly enhanced Raman signal on the other three substrates. Moreover, under the excitation of 785 nm, SERS substrates did not show a proper enhancement effect, whereas under 488 nm, 532 nm and 633 nm excitations, stronger enhancements are observed. This perfectly matches the wavelength requirement of the high temperature Raman experiments as high temperature reactions usually emit radiation in the red and infrared range. Also according to Leung's theory, generally the SERS effect of shorter excitation wavelengths is insensitive to temperature while the excitation wavelengths are above the surface plasmon resonance.15

image file: c7nr08631h-f2.tif
Fig. 2 Raman spectra of Rhodamine B on silicon wafer, Ag nanoparticles, Ag@10 nm SiO2 and Ag@50 nm SiO2 substrates under (a) excitation laser 488 nm; (b) excitation laser 532 nm; (c) excitation laser 633 nm; (d) excitation laser 785 nm.

For all excitations, the pure Ag shows the strongest enhancement for Rhodamine B, and the six typical Raman peaks of Rhodamine B are indexed on the Raman spectra in Fig. 2. As expected, the enhancement factor decreased with SiO2 thickness, and when the thickness of the SiO2 layer reaches 50 nm, the enhancement effect almost disappears because of the drastic decrease in the density of scattering spots as shown in Fig. 1e (inset). However, it is encouraging that the Ag@10 nm SiO2 (and also Ag@15 nm SiO2, not shown) substrate still exhibits the same order of magnitude for the enhancement at room temperature, which is efficient enough to detect many organic pollutants at low concentration. This result suggests that a 10 or 15 nm coating only reduces the enhancement factor by a few fold. Compared to orders of magnitude enhancement over the non-SERS conditions, this few fold decrease is nearly negligible. After confirming the SERS effect of these substrates, the Ag@10 nm SiO2 substrate was chosen to perform the following high-temperature experiment.

Fig. 3a shows the heating pattern used in this study and SEM characterization is performed at the same position at room temperature after annealing at different temperatures. Briefly the substrates are heated at 200, 400, 600, and 800 °C to investigate the thermal stability of the Ag@10 nm SiO2 substrate. In our previous work, a pure Ag film evolved from a continuous film to separate an island-like structure during the heating cycle starting from room temperature to 800 °C. In this work, however, SiO2 coated Ag nanoparticles successfully retain their spherical shape and size up to 800 °C (Fig. 3b–f), which confirms that the 10–15 nm SiO2 layer effectively improves the thermal robustness of Ag nanoparticles. Moreover, the morphology of Ag nanoparticles before and after annealing at 900 °C were also characterized by SEM, and the typical images are shown in Fig. S4a and b. The Ag nanoparticles were partially evaporated or aggregated during the temperature increase cycles and turned into a totally different morphology when observed after cooling to room temperature. Raman spectra of Rhodamine B before and after the heating process on Ag@10 nm SiO2 and pure Ag nanoparticle substrates (Fig. S4c and d) showed that the Ag@10 nm SiO2 substrate maintained stable SERS enhancement after high-temperature treatment.

image file: c7nr08631h-f3.tif
Fig. 3 (a) An experimental scheme recording the morphology change with elevated temperature; (b–f) representative SEM images of “Ag@10 nm SiO2” structure after each annealing cycle.

More precise and quantitative analysis of the difference between Ag@10 nm SiO2 before and after elevating temperature (800 °C) is carried out using TEM and the representative images are presented in Fig. 4c and d. Fig. 4d shows the typical morphology of Ag@10 nm SiO2 nanoparticles and Fig. 4c shows the TEM image of Ag@10 nm SiO2 nanoparticles after heating to 800 °C. In Fig. 4c and d, no clear difference in the morphology of the Ag core part is observed between the two samples. In these two observations, we successfully identified the contrast of core shell layer SiO2 even though the background SiO2 from the TEM grid is pretty strong. Furthermore, a uniqueness of this in-plane TEM is the capability of providing comprehensive structural information in a large area. The selected area electron diffraction (SAED) patterns of these two samples (Fig. 4c and d (inset)) are obtained with an aperture diameter of several micrometers, which collects information from more than 10[thin space (1/6-em)]000 particles. From SAED patterns, it is clearly revealed that the crystal structure of Ag is maintained after annealing at 800 °C.

image file: c7nr08631h-f4.tif
Fig. 4 SEM image of the “Ag nanoparticle” structure after annealing at (a) 900 °C and (b) 800 °C (particle size is too big for TEM characterizing); (c) representative TEM image of the premier “Ag@10 nm SiO2” structure after annealing at 800 °C and the corresponding selected area electron diffraction pattern; (d) representative TEM image of the premier “Ag@10 nm SiO2” structure and the corresponding selected area electron diffraction pattern; (e) histogram of diameter distribution of the Ag part in the “Ag@10 nm SiO2” structure before and after the annealing process and pure Ag particles after annealing at 800 °C and 900 °C.

Fig. 4a and b show the SEM images of pure Ag nanoparticles after annealing at 900 °C and 800 °C with a protective Ar/H2 (3%) atmosphere. The size of Ag nanoparticles after annealing is too large to be observed using TEM. For annealing after 800 °C, the morphology of the substrate significantly changed and many large nanoparticles appeared after high temperature treatment. For the 900 °C treatment, the morphology of the substrate is totally changed and the particles are larger than the original particles. From the comparison of Fig. 4a and b, annealing at 900 °C has more notable influence on the morphology of Ag nanoparticles because the mobility of the surface atom at 900 °C is higher than 800 °C. The size distribution of the Ag core in Ag@10 nm SiO2 nanoparticles is mainly concentrated below 20 nm, as shown in Fig. 4e, and the average diameter of the Ag core is 5.2 nm. Similarly, the diameter of the Ag core in Ag@10 nm SiO2 nanoparticles after elevating to 800 °C is mostly below 20 nm, and the average size is 7.1 nm. The comparison of two histograms clarified that the heating process has less impact on the particles whose diameter is in the 10–20 nm range, but small particles with diameters below 10 nm migrated and merged with large particles. This could also be directly evidenced by the empty SiO2 core shell in the low magnification TEM image of Ag@10 nm SiO2 nanoparticles after heating to 800 °C (Fig. S5). In contrast, without the protection of SiO2, the size of the Ag nanoparticles is widely distributed after high temperature treatment and the average size increases to 15.7 nm (800 °C) and 58.9 nm (900 °C). The TEM characterization indicates that the thermally stable SiO2 layer coated nanoparticles show a potential to serve as effective operando SERS substrates at high temperature.

After confirming the SERS enhancement of the Ag@SiO2 structure at room temperature and its thermal stability at high temperature, we present a preliminary exploration on the application of this high temperature SERS substrate to investigate the high-temperature SERS spectra of SWNTs. A horizontally aligned SWNT film synthesized by the alcohol catalytic chemical vapor deposition method is transferred on to a Ag@10 nm SiO2 substrate with the assistance of poly(methyl methacrylate). Fig. 5a shows comparisons of SERS (red) and non-SERS (blue) spectra of SWNTs recorded from 30 °C up to 800 °C under 488 nm wavelength excitation. A protective Ar/H2 atmosphere is introduced in the reaction chamber during the entire measurement to prevent the oxidation of SWNTs. In Fig. 6a, two representative peaks of SWNTs (G and D bands) are clearly enhanced on the SiO2 coated Ag substrate (red curves). Though the overall intensity decreased at elevated temperatures, the enhancement was successfully retained up to 800 °C.

image file: c7nr08631h-f5.tif
Fig. 5 (a) Non-SERS spectra of HA-SWNT on the SiO2 substrate and SERS spectra of the same HA-SWNT on the “Ag@10 nm SiO2” substrate at different temperatures (excitation laser 488 nm); (b) the G-band position of HA-SWNT on non-SERS and SERS substrates at different temperatures; (c) SERS enhanced factors of HA-SWNT at different temperatures.

image file: c7nr08631h-f6.tif
Fig. 6 (a) Temperature-dependent in situ Raman spectra of nanodiamonds during the thermally annealed process on the Ag@10 nm SiO2 substrate; (b) the G-band positions from analysis of the temperature-dependent in situ Raman spectra as a function of temperature; (c) the ratio of the intensities of D-band to G-band from the analysis of the temperature-dependent in situ Raman spectra as a function of temperature.

The G band position of SWNTs on the SERS (red) and non-SERS (blue) area vs. temperature is plotted in Fig. 5b. There are at least two main changes between the two Raman spectra. First, the G band position of the SERS and non-SERS spectra shares the same trend that down shifts lower wavenumbers, which has been well verified in previous studies and can be illustrated by the tube structure expansion and C–C bond softening at high temperature.30–32 Secondly, there are small shifts (∼3 cm−1) in the G band to a lower wavenumber on the Ag@10 nm SiO2 substrate compared to the non-SERS part, which is one typical feature of the SERS effect. Moreover, the enhancement factor vs. temperature is shown in Fig. 5c, in which clear enhancements can be observed at all temperatures. Besides, there is no obvious drop in the enhancement factor with temperature although the enhancement factor of around 3 is quite small. Here we claim that this enhancement factor for SWNTs is the entire effect for all SWNTs due to their 1D geometric characteristics, and the intrinsic enhancement at a local hot spot should be much higher than this value.

Much more notable enhancements were observed when a nano-diamond, a 0D material, was tested on this SiO2 coated Ag substrate. In this experiment, nanodiamonds in ethanol solution were dropped on the Ag@10 nm SiO2 SERS substrate and a high temperature annealing process was performed under a Ar/H2 (3%) atmosphere. The representative in situ Raman spectra of nanodiamonds at different elevated temperatures on the SERS substrate are shown in Fig. 6a, and for comparison the Raman spectra under the same conditions on the normal silicon wafer are shown in Fig. S6a. Before enhancement, there are almost no obvious structure-related fingerprint peaks in the Raman spectra of nanodiamonds on the normal silicon wafer (non-SERS spectrum in Fig. 6a). However, there is a SERS enhancement of nanodiamonds on the Ag@10 nm SiO2 substrate from room temperature to harsh elevated temperature until 800 °C.

Generally, it is assumed that there are mixtures of carbon sp2 and sp3 bonds in nanodiamonds and thermal treatment would change the structure of amorphous carbon to a graphitic structure. The G band (1560 cm−1) positions and the intensity ratio of the D band (1350 cm−1) to G band (ID/IG) from analysis of the temperature-dependent in situ Raman spectra of the thermally annealed nanodiamonds at different temperatures are shown in Fig. 6a and b. During the annealing process, nanodiamonds graphitized gradually from 200 °C, which is revealed by the positions of G-bands that shift slightly to a higher wave number and the ID/IG ratio that starts to increase at the same temperature. These results indicate that the interconvertion of sp3 to sp2 in nanodiamonds began at 200 °C and resulted in a stable graphite-like phase at 600 °C. This Ag@10 nm SiO2 substrate provides a possibility to precisely observe the annealing process of nanodiamonds and other chemical reactions at elevated temperatures.

Finally, to illustrate the underlying enhancement mechanism from a theoretical aspect and better understand the influence of the SiO2 coating on the distribution of electric field, we perform FDTD method33,34 to simulate the intensity of the relative electric field. The FDTD method provides a convenient, systematic and general approach for calculating the optical response of plasmonic nanostructures of arbitrary symmetry and geometry to an incident light wave, which transfers the differential equations to difference equations. We adopt a 30 nm Ag sphere placed on the SiO2 flat substrate as a basic model (Fig. S7a). The plane wave is normally incident on the substrate with p-polarization (inset of Fig. S7a). Two Ag spheres with a gap size of 10 nm, an Ag sphere with 5 nm SiO2 layer capping, two 5 nm SiO2 coated Ag spheres, and a random distribution of Ag spheres with and without the 5 nm SiO2 coated Ag spheres are simulated in this work.

Fig. 7a shows the FDTD simulated electric-field distribution of a single Ag sphere and Fig. 7b gives the electric-field distribution in the gap between the two Ag nanospheres. From Fig. 7a and b, it is concluded that the strongest electric field enhancement originates from the gap between nanospheres and the “hot spot” in the gap region is highly localized on the surface of each nanosphere. In the random distribution of Ag nanospheres (Fig. 7c), the smaller gaps generate a stronger electric field. Fig. 7d, e and f show the similar simulated results for the 5 nm SiO2 coated nanospheres. The overall distribution of the electric field is not affected by the SiO2 coating, however the strongest “hot spots” are physically inaccessible because they are excited at the interface between Ag and SiO2. This may explain the decrease in the SERS signal when compared to the original bare Ag nanospheres. Also SiO2 coating reduces the inter-distance between all particles, so that some gaps will be closed and lose their activity as hot spots. On average, a smaller number of hot spots may be available after the SiO2 coating and so the SERS signal should decrease. Nonetheless, the remaining electric field originating from the SiO2 coated nanospheres could still perform a noticeable enhancement in the Raman scattering in this work and we demonstrate that this structure can enhance the Raman signal stably at high temperatures.

image file: c7nr08631h-f7.tif
Fig. 7 Simulation results of electric field distribution by the FDTD method of (a) a single pure Ag nanoparticle; (b) double pure Ag nanoparticles; (c) random distribution of pure Ag particles with different sizes; “Ag@5 nm SiO2” structure (d) single particle; (e) double particles; (f) random distribution of particles with different sizes.


In conclusion, we presented a systematic study on the fabrication, characterization, and the SERS effect of SiO2 coated Ag nanoparticles, aiming to design a thermally stable and versatile substrate for monitoring high temperature catalytic reactions. A SiO2 coating of 10–15 nm is found to be able to significantly enhance the thermal stability of Ag particles without much sacrificing their SERS effect. TEM and SEM characterization after testing the structure at elevated temperatures shows that the Ag coated by SiO2 particles were verified to maintain stability and retain the same size and morphology at temperatures up to 900 °C. The monitoring of the annealing process of nano-diamonds revealed the interconverting of C–C bonds and also explored the possibility of using this substrate to observe other chemical reactions. Finally, the FDTD simulation proved that the coating SiO2 layer did not change the distribution of electric field but only physically trapped the most enhanced spot. We claim this work is a first experimental proof that the high temperature SERS effect can be preserved and applied in a chemical reaction at temperature above 500 °C, and emphasize that this substrate can be an efficient and versatile SERS substrate to monitor processes such as etching, annealing and growth processes for various material systems (particularly 0D and 2D nanomaterials), possibly including semi-conducting nano-particles, graphene, MoS2 and/or other 2D materials.

Experimental methods

Fabrication of SERS substrates

A thin film of silver (nominal thickness 5 nm) was deposited on an n-type Si substrate with a 100 nm thick SiO2 layer (SUMCO Technology Co., Ltd) by a thermal evaporation technique at a background vacuum level of 10−3 Pa. After silver deposition, the substrate was annealed at 600 °C for 5 min with the protection of Ar diluted H2 (3%) to form Ag nanoparticles. Finally, SiO2 layers of different thicknesses were deposited on the Ag nanoparticles by sputtering. The morphology and structure of Ag nanoparticles and Ag coated by SiO2 layers were characterized by optical microscopy (dark field mode), SEM (HITACHI-S4800) and TEM (JEOL 2010F).

Raman spectroscopy

The SERS spectra were obtained with a Renishaw InVia micro Raman system equipped with a ×20 objective lens. Four different laser lines (488 nm, 532 nm, 633 nm, and 785 nm) were used as the excitation light sources. An ethanol solution containing a 5.3 × 10−4 M rhodamine B (C28HClN2O3, Wako, CI 45179) was applied to the SERS substrate to verify the enhancement at room temperature. Nanodiamonds were sonicated in the ethanol solution for 30 min and then drop-cast onto the substrate to perform high temperature SERS. Horizontally aligned SWNTs were synthesised using the alcohol catalytic chemical vapour deposition method35,36 on r-cut crystal quartz substrates, and then transferred onto the SERS substrates via poly(methyl methacrylate) (PMMA) thin films.37

Simulation model

The 3D finite-difference time-domain technique is used to compute the distribution of the electromagnetic fields of five models and explore the SERS mechanism of the core shell particles. Particularly, the effect of the gap between particles is investigated by reporting the field distribution for the dimer arrangement with and without the SiO2 protective coating. The reported distributions are given for the wavelengths corresponding to the maximum of the electric field component aligned with the dimer symmetry axis.

Conflicts of interest

There are no conflicts to declare.


Part of this work is financially supported by the JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, JP15K17984, JSPS-NSFC bilateral joint research projects and the IRENA Project by JST-EC DG RTD, Strategic International Collaborative Research Program, SICORP. Part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and by the “Nanotechnology Platform” (project No. 12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Notes and references

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Electronic supplementary information (ESI) available: IV curve of the SERS substrate, comparison of SEM images and the SERS effect between original Ag nanoparticles and Ag nanoparticles after annealing at 900 °C, temperature-dependent in situ Raman spectra of nanodiamonds during a thermally annealed process on a silicon wafer, and schematic illustration of 3D FDTD simulation mode. See DOI: 10.1039/c7nr08631h

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